The present invention relates to the removal of pollutants from gases, and more particularly to the removal of nitrogen oxides, such as NO and NO2, and other pollutants including particulates from exhaust gases or other industrial gases such as produced by internal combustion engines using a plasma-assisted catalytic surface, and to industrial processes generating such gases.
Carbonaceous fuels are burned in internal combustion engines and other equipment, including boilers, furnaces, heaters, incinerators, and the like (i.e., in a wide variety of industrial processes). Excess air frequently is used to complete the oxidation of combustion byproducts such as carbon monoxide (CO), hydrocarbons, and soot. High temperature combustion using excess air, however, tends to generate nitrogen oxides (often referred to as NOx). In addition, a number of fossil fuel combustion sources result in polluted exhaust streams. These sources include internal combustion engines such as diesel, natural gas, and lean burn gasoline as well as external combustion sources such as boilers, incinerators, and other NOx, particulate and hydrocarbon containing streams. The polluted exhaust streams from such sources also may contain high O2 (0-18%) levels. Reducing NOx can be particularly difficult for such gases containing high O2 levels.
Emissions of NOx include nitric oxide (NO) and nitrogen dioxide (NO2). During combustion, it is believed that free radicals of nitrogen (N2) and oxygen (O2) combine chemically primarily to form NO at high temperatures. Mobile and stationary combustion equipment are concentrated sources of NOx emissions. If discharged to the environment, NO emissions oxidize to form NO2, which tends to accumulate excessively in many urban areas. In sunlight, the NO2 reacts with volatile organic compounds to form ground level ozone, eye irritants, and photochemical smog. These adverse effects have prompted extensive efforts for controlling NOx emissions. Despite advancements in fuel and combustion technology, ground level ozone concentrations still exceed federal guidelines in many urban areas. Under the Clean Air Act and its amendments, these ozone nonattainment areas must implement strategies for low NOx, which can only be attained by exhaust aftertreatment.
Exhaust aftertreatment techniques tend to remove NOx using various chemical or catalytic methods. Such methods are known in the art and typically involve either reduction to N2 or oxidation to NO2 and subsequently to HNO3. The former reduction processes generally involve either nonselective catalytic reduction (NSCR), selective catalytic reduction (SCR) or selective noncatalytic reduction (SNCR). Alternatively, NO may be oxidized to NO2 for removal by wet scrubbers. Such aftertreatment methods typically require some type of additional reactant to remove the NOx emissions. The use of these reactants often results in safety problems in addition to the added cost of the reactant. It would be more desirable to utilize reduction as opposed to oxidation because reduction of NO results in benign N2, while oxidation or NO results in NO2. Furthermore, it would be desirable to achieve reduction of NO to N2 without the use of additional reactants or additives.
Although a number of different catalytic and non-catalytic postcombustion technologies have been used for NO removal, none have been able to convert NO to N2 to an acceptable degree in the presence of large amounts of O2 and/or H2O. Additives such as nitrogen based chemicals (NH3) and hydrocarbons also have been used to yield NOx reduction to N2, but such techniques tend to result in higher cost and are undesirable as they tend to present storage, safety, and by-product slippage problems.
Conventional catalytic technologies for the selective removal of NOx tend to operate at temperatures between 600-1000xc2x0 F. and require the use of additives such as NH3 (toxic) or hydrocarbons, often with undesirable by-products and safety concerns. Non-catalytic technologies tend to require much higher temperatures (above 1300xc2x0 F.), requiring accessory equipment to increase its temperature and needing toxic additives such as NH3.
The use of non-thermal plasmas for NOx and particulate removal at low temperatures is described in the literature. Without being bound by theory, a non-thermal plasma consists of high energy electrons that are highly reactive, but thermally cool (hence xe2x80x9cnon-thermalxe2x80x9d). It is believed that these reactive electrons collide with the primary components of the polluted gas stream to form the active species in-situ, which in turn may remove NOx and particulate emissions.
Attempts to remove NOx from exhaust gases using various types of plasma reactors has been explored. A variety of reactors, which differ primarily in the mode of generating electrons through an electrical discharge, have been used for NOx removal. These include the following: (1) corona (DC or pulsed); (2) dielectric barrier discharge; and (3) dielectric packed bed reactor. In general, the polluted gas stream is passed through each of the reactors in which a non-thermal plasma is generated, leading to the in-situ formation of the desired active species. In the presence of O2 (as in typical diesel exhaust), studies conducted to date using these discharge reactors for NOx removal have reported the oxidation of NO to NO2 with very poor selectivity to the desired species, N2.
Mathur et al. (U.S. Pat. Nos. 5,240,575 and 5,147,516) and Breault et al. (U.S. Pat. No. 5,458,748) have discussed using a corona as well as a xe2x80x9ccatalyzedxe2x80x9d corona reactor to treat simulated exhaust. The general thrust of such disclosures is that NO is primarily removed by oxidation to NO2 in the presence of O2, with subsequent absorption as HNO3. A number of prior art studies referenced in Mathur and Breault also describe the removal of NO by oxidation to NO2. Other studies, such as Penetrante et al. (NOx Reduction by Compact Electron Beam Processing, Proceedings of the 1995 Diesel Engine Emissions Reduction Workshop, University of California, San Diego, Jul. 24-27, 1995, p. IV75-85), Wallman et al. (Nonthermal Aftertreatment of Diesel Engine Exhaust, Proceedings of the 1995 Diesel Engine Emissions Reduction Workshop, University of California, San Diego, Jul. 24-27, 1995, p. V33-39), Civitano et al. (Flue Gas Simultaneous DeNOx/DeSOx by Impulse Corona Energization, 3rd International Conference on Electrical Processing, 1987), Mizuno et al. (Application of Corona Technology in the Reduction of Greenhouse Gases and Other Gaseous Pollutants., Non-Thermal Plasma Techniques for Pollution Control-Part B: Electron Beam and Electrical Discharge Processing, (Edited by B. M. Penetrante and S. E. Schultheis), Springer-Verlag, Heidelberg, 1993), and Fujii et al. (Simultaneous Removal of NOx, COx, SOx and Soot in Diesel Engine Exhaust., Non-Thermal Plasma Techniques for Pollution Control-Part B: Electron Beam and Electrical Discharge, (Edited by B. M. Penetrante and S. E. Schultheis), Springer-Verlag, Heidelberg, 1993, 257-279), which used a diesel film present the shift in the NO removal to NO2 instead of the desired product N2, with the introduction of less than 2% O2 in the feed gas.
Similarly, Gentile et al. (Microstreamer Initiated Advection in Dielectric Barrier Discharges for Plasma Remediation of NxOy: Single and Multiple Streamers, Proceedings of the 1995 Diesel Engine Emissions Reduction Workshop, University of California, San Diego, Jul. 24-27, 1995, p. V45-56, and Microstreamer Dynamics During Plasma Remediation of NO using Atmospheric Pressure Dielectric Barrier Discharges: Single and Multiple Streamers, Proceedings of the Eight ONR Propulsion Meeting, San Diego, Calif., 1995, p. 64-69) used a dielectric barrier discharge, resulting in NO removal by oxidation to NO2.
The average kinetic energy of the electrons in a conventional gas phase plasma discharge (such as described in the above studies) is less than 10 eV. Under such conditions, and in the presence of high O2 concentrations (e.g., 2-18%), Penetrante has shown that O2 is preferentially dissociated compared to N2, resulting in a low selectivity to N2; the predominant pathway being the undesired conversion of NO to NO2 and further to HNO3.
Bayliss et al.(U. S. Patent No. 5,440,876) and Fanick et al. (Reduction of Diesel NOx/Particulate Emissions using a Non-thermal Plasma, Proceedings of the 1995 Diesel Engine Emissions Reduction Workshop, University of California, San Diego, Jul. 24-27, 1995, p. V57-67) describe a gas purification device which uses a high dielectric ferroelectric material packed between two electrodes to demonstrate oxidation of particulates to CO2 in diesel exhaust. Though not clearly mentioned, this particulate trap also results in NO oxidation to NO2. The pellets are listed to be preferentially Pb or Ba niobate, titanate, or zirconate. Thus, the prior art literature does not provide a method for the selective reduction of NOx to N2 without the use of additives, in the presence of high O2 concentrations.
Other studies describe the use of additives such as hydrocarbons and NH3 to achieve NOx reduction to N2. Vogtlin et al. (Pulsed Corona Discharge for Removal of NOx from Flue Gas., Non-Thermal Plasma Techniques for Pollution Control-Part B: Electron Beam and Electrical Discharge, (Edited by B. M. Penetrante and S. E. Schultheis), Springer-Verlag, Heidelberg, 1993, 187-198) and Chess et al. (Plasma versus Thermal Effects in Flue Gas NOx Reduction Using Ammonia Radical Injection, J. Air and Waste Manage. Assoc., 45, 627-632) describe the use of hydrocarbons and NH3 respectively, but these require supplemental equipment often resulting in safety, storage, and by-product slippage concerns. Using additives is clearly costly, inconvenient, and commercially impractical for NOx removal.
Despite such extensive prior art activities, a need remains for systems and methods of selectively reducing NOx to N2 and oxidation of particulates and hydrocarbons to CO2 in O2-containing polluted streams without the use of supplemental reactants or additives.
The present description provides methods and systems for removing NOx, particulates, and hydrocarbons from O2 rich pollutant streams, using a non-thermal plasma generated between two electrodes with a catalytic packing between the electrodes. The combination of a plasma and the catalytic packing selectively catalyzes and enhances the reduction of NOx to N2 and oxidation of particulates and hydrocarbons to CO2.
The present invention utilizes desirable combinations of materials and plasma to selectively and catalytically reduce NOx to N2, without supplemental additives. Preferably, the desired catalytic materials, as more described herein, consist of materials such as metal promoted or unpromoted solid oxide catalysts having properties to scavenge oxygen and/or to otherwise result resulting in preferential and selective NOx reduction. Also, preferably, when the desired plasma is combined with the desired materials, this combination results in and drives the selective NOx reduction process. In certain preferred embodiments, the catalyst is desirably formed in a monolithic or honeycomb manner, and, in certain internal combustion engine-related embodiments, it is constructed to operate while consuming minimal engine power, while reducing NOx to a desirable level.
Accordingly, it is an object of the present invention to provide an apparatus and a method for selectively removing NOx, particulate, and hydrocarbons from O2 rich pollutant streams, including without the need for supplemental additives.
It is a further object of the present invention to provide an apparatus and a method for removing NOx, particulate, and hydrocarbon by using a non-thermal plasma generated between two electrodes with a catalytic packing between the electrodes.
It is another object of the present invention to provide an apparatus and a method in which the catalytic packing selectively catalyzes the reduction of NOx to N2 and oxidation of particulates and hydrocarbons to CO2.
Finally, it is an object of the present invention to provide an apparatus and a method that may be practically applied to a variety of combustion and effluent gas sources, including internal combustion engines, burners, boilers, and other combustion and industrial processes.